System and method for dual mode control of a turbogenerator/motor

ABSTRACT

A method of controlling a permanent magnet turbogenerator/motor includes providing a protected load connected in parallel with the turbogenerator/motor through a pulse width modulated inverter configured in a first operating mode to supply controlled current from the turbogenerator/motor to a utility electrical power source, and selectively connected to the utility electrical power source through an isolation device, monitoring the utility electrical power source, and automatically disconnecting the protected load from the utility electrical power source while reconfiguring the pulse width modulated inverter in a second operating mode to supply controlled voltage to the protected load when a fault is detected in the utility electrical power source. The present invention provides an external or integrated dual mode controller which can be used to provide automatic transitions of a turbogenerator/motor between grid-connected modes where a PWM inverter provides a controlled AC current source to stand-alone modes where the PWM inverter provides a controlled AC voltage source to a protected load. Three phase rotation sequence can be set to positive or negative rotation, or be set to the last measured rotation for the utility grid to enable automatic transitions from stand-alone to grid-connect modes. In addition, the present invention provides for detecting a reference phase angle of the three-phase power provided by a utility grid such that gradual synchronization/resynchronization of a protected parallel connected load can be performed by appropriately controlling the PWM inverter prior to re-connection to the utility grid.

This is a continuation application of U.S. Ser. No. 09/644,527 filed onAug. 23, 2000 now U.S. Pat. No. 6,410,992, now allowed.

TECHNICAL FIELD

The present invention relates to a system and method for controlling aturbogenerator/motor to provide automated or semi-automated transitionsbetween grid following and stand-alone operating modes.

BACKGROUND ART

A permanent magnet generator/motor generally includes a rotor assemblyhaving a plurality of equally spaced magnet poles of alternatingpolarity around the outer periphery of the rotor or, in more recenttimes, a solid structure of samarium cobalt or neodymium-iron-boron. Therotor is rotatable within a stator which generally includes a pluralityof windings and magnetic poles of alternating polarity. In a generatormode, rotation of the rotor causes the permanent magnets to pass by thestator poles and coils and thereby induces an electric current to flowin each of the coils. Alternatively, if an electric current is passedthrough the stator coils, the energized coils will cause the rotor torotate and thus the generator will perform as a motor.

One of the applications of a permanent magnet generator/motor isreferred to as a turbogenerator which includes a power head mounted onthe same shaft as the permanent magnet generator/motor, and alsoincludes a combustor and recuperator. The turbogenerator power headwould normally include a compressor, a gas turbine and a bearing rotorthrough which the permanent magnet generator/motor tie rod passes. Thecompressor is driven by the gas turbine which receives heated exhaustgases from the combustor supplied with preheated air from therecuperator.

A permanent magnet turbogenerator/motor can be utilized to provideelectrical power for a wide range of utility, commercial, and industrialapplications. While an individual permanent magnet turbogenerator mayonly generate 24 to 75 kilowatts, powerplants of up to 1000 kilowatts orgreater are possible by linking numerous permanent magnetturbogenerator/motors together. Potential applications for theselightweight, low noise, low cost, environmentally friendly, andthermally efficient units include standby power, peak load shavingpower, and remote location power, among others.

When operating in a grid connected mode, the turbogenerator system isgenerating power in parallel with a utility grid. In this mode, thesystem may act as a current source, or alternatively as a voltage sourcewith series impedance. When generating power in parallel with theutility, various protective functions or features should be provided toassure the safety of utility workers and others who interface with theutility grid. For example, features should be provided to disconnect theturbogenerator system from the utility grid when abnormal conditionsoccur on the grid system, particularly when the section of the grid towhich the turbogenerator system is connected becomes isolated from theremainder of the utility system. This condition is sometimes referred toas “islanded operation” and should be avoided because utility workersnormally expect the voltage to collapse when they disconnect a gridsection from the remainder of the utility. If this does not occur asexpected, and the utility workers do not follow proper safety proceduressuch as measuring the voltage and applying grounds before touchingconductors, islanded operation may result in a safety risk. Anothersafety risk is that a generator will start up and energize ade-energized section of the grid with which utility workers may be incontact.

When operating in stand alone mode, the turbogenerator system isgenerating power to supply loads that are isolated from the utilitygrid. The system will generally operate as a voltage source in this modealthough other configurations are possible. In stand alone mode, theprotective features used in grid connect mode should be disabled becausethe system is essentially operating in an islanded condition andenergizes a de-energized line. However, various other features may bedesirable, such as avoiding energizing a line that is already energizedso that stand alone controls can not be activated when connected to autility grid.

A gas turbine is an inherently limited thermal machine from thestandpoint of its ability to change rapidly from one load state to adifferent load state. In terms of accepting increased loading, the gasturbine has a limited capability or slew rate which may depend upon theparticular application. As one example, a turbogenerator may be able tosupport an increasing load at a rate of about two (2) kilowatts persecond. This creates problems in many stand-alone applications where theload may be essentially instantaneously applied in as little as aboutone millisecond. Similar limitations apply to decreasing loads as well.When operating in a self-sustained manner, the gas turbine has a verylarge amount of stored energy, primarily in the form of heat stored inthe associated recuperator. If the load is removed from the gas turbinetoo quickly, this stored energy could result in overspeeding theturbine.

As such, various energy storage and discharge systems have beendeveloped. The energy storage and discharge system includes an ancillaryelectric storage device, such as a battery, connected to the generatorcontroller through control electronics. Electrical energy can flow fromthe ancillary electric storage device to the turbogenerator controllerduring start up and increasing load and vice versa during self-sustainedoperation of the turbogenerator. To start the turbine, amicroprocessor-based inverter connects to and supplies fixed current,variable voltage, variable frequency, AC power to the permanent magnetturbogenerator/motor, driving the permanent magnet turbogenerator/motoras a motor to accelerate the gas turbine. When utility power isunavailable, the ancillary electric storage device can provide the powerrequired to start the turbogenerator. As the turbine accelerates to anappropriate light-off speed, spark and fuel are introduced andself-sustaining gas turbine operating conditions are reached. Whentransitioning between stand alone and grid connect operating modes,proper phase synchronization may be necessary to avoid damagingequipment associated with the protected load.

When a load transient occurs in stand alone mode, the gas turbine engineand the ancillary electric storage device provide the power required tosuccessfully meet the transient. The output power control regulates aconstant AC voltage and any load placed on the output will immediatelyrequire more power to maintain the same level of AC voltage output. Asthis occurs, the internal DC bus may start to droop. In response, theancillary electric storage device control draws current out of thestorage device to regulate the DC bus voltage. As turbogenerator systempower output increases, the gas turbine engine controls respond bycommanding the gas turbine engine to a higher speed. When the powergenerated by the gas turbine engine meets or exceeds the current load,the ancillary storage device starts to draw power from the DC bus forfuture storage. Proper management of the ancillary storage device isnecessary to assure stored energy availability for use during standalone starts where utility power is not available, during transitionsbetween grid connect and stand alone operating modes, and to accommodatetransient load increases while maintaining acceptable energy storagecomponent life.

DISCLOSURE OF INVENTION

The present invention provides systems and methods for controlling aturbogenerator to facilitate switching between a grid followingoperating mode and a stand-alone operating mode. A dual mode controllermay by an external controller which communicates with the primaryturbogenerator/motor controller via an appropriate communicationsinterface to selectively and automatically reconfigure a PWM inverterfrom a grid-connect mode to a stand-alone mode based on availability orcondition of utility grid power. Control commands and data/statusinformation may be communicated between the turbogenerator/motorcontroller and the dual mode controller via serial communicationinterfaces (RS-485, Ethernet), logic signals, relays, contacts, and thelike. In one embodiment, the dual mode controller is integrated into theturbogenerator/controller and may include the same or similar contactsfor backward compatibility.

The present invention provides a dual mode controller for aturbogenerator which includes a rotation sequence selector which canautomatically monitor the rotation sequence of three phase power from autility grid and use the same rotation sequence when transitioningbetween grid-connect and stand-alone mode modes. Alternatively, or incombination, the present invention allows a user/operator to select arotation sequence while operating in a manual stand-alone mode which mayinclude a positive or negative sequence, or may use the last measuredsequence for the utility grid. A default rotation sequence, eitherpositive, negative, or grid, is also provided for automatic transitionsbetween the grid-connect and stand-alone mode operation. In oneembodiment, the present invention also provides for phase referenceangle synchronization to synchronize the reference angle in stand-alonemode with the utility grid prior to completing the transition togrid-connect mode.

The present invention preferably includes an auxiliary electrical powerstorage and discharge system, such as one or more batteries, forproviding a stand-alone start of the turbogenerator/motor withoutconnection to a utility grid, for providing power to service transientloads, and/or for providing power to a protected load during transitionsbetween grid-connect and stand-alone operating modes. Appropriatemanagement of the auxiliary electrical power and discharge system mayinclude periodic top-up charges, equalization charges, and recharges toassure power availability.

The present invention includes various safety features whiletransitioning between grid connect and stand-alone operating modes. Forexample, if an attempt is made to operating in grid connect mode whenthe connected load is actually a stand-alone, then the system inhibitsstarting to avoid safety hazards. If an attempt is made to operate instand-alone mode when the system is connected so the utility grid andthe grid is energized, the system will not start. The invention preventsthe system from being configured in the stand-alone mode when it isconnected to a utility grid by providing an appropriate lock-out system.An isolation device contains an auxiliary low voltage contact thatalways has the same position as the main power contacts. The systeminterfaces with this contact via stand-alone and grid-connect enablelines. When the switch is closed, the stand-alone enable is clear andthe grid-connect enable is set. The invention opens the isolation deviceif the turbogenerator system enters the stand-alone state where theoutput is energized by using state feedback signals provided by theturbogenerator.

The present invention provides a number of advantages relative to priorart strategies. For example, the present invention can provide automaticor semiautomatic transitions between grid-connect and stand-aloneoperating modes with only momentary breaks or interruptions in supplyingpower to a protected load. The ability for rotation selection control ofthree-phase power generation in addition to phase synchronizationprovides for automatic transitions from stand-alone modes togrid-connect operating modes. In addition, rotation sequence control mayby used to prevent equipment damage otherwise caused by a rotationsequence reversal when switching to stand-alone mode.

The above advantages and other advantages, objects, and features of thepresent invention, will be readily apparent from the following detaileddescription of the best mode for carrying out the invention when takenin connection with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a partial cut-away view of a low emissionsturbogenerator/motor for use with a dual mode control system or methodaccording to the present invention;

FIG. 2 is a block diagram of a control system for dual mode operation ofa turbogenerator/motor according to the present invention;

FIG. 3 is a block diagram illustrating additional details of a dual modecontroller for one embodiment of a dual mode turbogenerator/motor systemaccording to the present invention;

FIG. 4 is a simplified flowchart illustrating the rotation sequencemeasurement and control logic used by a dual mode turbogenerator/motorsystem according to the present invention.

FIG. 5 is a simplified flow chart illustrating dual mode control of atransition from a grid-connect mode to a stand-alone mode according tothe present invention;

FIG. 6 is a simplified flowchart illustrating dual mode control withoutresynchronization and with a stop and automatic restart for a transitionfrom stand-alone mode to a grid-connected mode according to the presentinvention;

FIG. 7 is a simplified flowchart illustrating dual mode control withoutresynchronization and without a stop and automatic restart for atransition from stand-alone mode to a grid-connected mode according tothe present invention;

FIG. 8 is a simplified flowchart illustrating dual mode control withresynchronization and without a stop and automatic restart for atransition from stand-alone mode to a grid-connected mode according tothe present invention;

FIG. 9 is a state transition diagram illustrating operation of a dualmode control system in the grid-connect mode including states fortransitioning from grid-connect mode to stand-alone mode according tothe present invention; and

FIG. 10 is a state transition diagram illustrating operation of a dualmode control system in the stand-alone mode including states fortransitioning from the stand-alone mode to the grid-connect modeaccording to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

A permanent magnet turbogenerator/motor 10 is illustrated in FIG. 1 asan example of a turbogenerator/motor for use with a system or method fordual mode control of the present invention. Permanent magnetturbogenerator/motor 10 generally comprises a permanent magnet generator12, a power head 13, a combustor 14 and a recuperator (or heatexchanger) 15. Permanent magnet generator 12 includes a permanent magnetrotor or sleeve 16, having a permanent magnet disposed therein,rotatably supported within a stator 18 by a pair of spaced journalbearings. Radial stator cooling fins 25 are enclosed in an outercylindrical sleeve 27 to form an annular air flow passage which coolsthe stator 18 and preheats the air passing toward power head 13.

Power head 13 of the permanent magnet turbogenerator/motor 10 includescompressor 30, turbine 31, and bearing rotor 36 through which tie rod 29passes. Compressor 30, having compressor impeller or wheel 32 whichreceives preheated air from the annular air flow passage in cylindricalsleeve 27 around permanent magnet stator 18, is driven by turbine 31having turbine wheel 33 which receives heated exhaust gases fromcombustor 14 supplied with air from recuperator 15. Compressor wheel 32and turbine wheel 33 are rotatably supported by bearing shaft or rotor36 having radially extending bearing rotor thrust disk 37. Bearing rotor36 is rotatably supported by a single journal bearing within the centerbearing housing while bearing rotor thrust disk 37 at the compressor endof bearing rotor 36 is rotatably supported by a bilateral thrustbearing. Bearing rotor thrust disk 37 is adjacent to the thrust face atthe compressor end of the center bearing housing while a bearing thrustplate is disposed on the opposite side of bearing rotor thrust disk 37relative to the center housing thrust face.

Intake air is drawn through permanent magnet generator 12 by compressor30 which increases the pressure of the air and forces it intorecuperator 15. In recuperator 15, exhaust heat from turbine 31 is usedto preheat the air before it enters combustor 14 where the preheated airis mixed with fuel and burned. The combustion gases are then expanded inturbine 31 which drives compressor 30 and permanent magnet rotor 16 ofpermanent magnet generator 12 which is mounted on the same shaft asturbine 31. The expanded turbine exhaust gases are then passed throughrecuperator 15 before being discharged from turbogenerator/motor 10.

Turbogenerator/motor 10 produces voltage at variable voltage andvariable frequency depending on the output power. However, the protectedloads supplied by turbogenerator/motor 10 typically require a fixedsupply voltage and frequency. As such, an electronic converter is usedto match the variable voltage, variable frequency output ofturbogenerator/motor 10 to the fixed voltage, fixed frequencyrequirements of the protected loads.

A functional block diagram of the interface between generator controller40, permanent magnet turbogenerator/motor 10, and a dual mode controller90 according to the present invention is illustrated in FIG. 2. Whileillustrated as a separate functional block in the embodiment of FIG. 2,dual mode controller 90 may either be integrated as one or moresubassemblies within the same physical packaging as generator controller40, or be a stand alone controller which communicates via one or morelogic/enable signals with generator 40 through connection 93. In onepreferred embodiment according to the present invention, dual modecontroller 90 communicates a grid connect enable signal, a stand-aloneenable signal, a battery start signal and a system start/stop signal togenerator controller 40 via appropriate logic signals as described ingreater detail below. Generator controller 40 supplies state indicationsto dual mode controller 90 which may include standby, run, load, fault,stand-alone, and/or grid connect mode active states as illustrated anddescribed with reference to FIGS. 5-10.

Dual mode controller 90 receives power from a source such as a utilitygrid. Dual mode controller 90 is operative to selectively connect powerto generator 40 and parallel connected protected load 92 to the utilityelectrical power source 41 using a controllable isolation device 91.Dual mode controller 90 is operative to selectively connect power togenerator 40 and parallel-connected protected load 92 using acontrollable isolation device 91. Controllable isolation device 91 maybe implemented using a contactor or a motorized circuit breaker, forexample. Dual mode controller 90 operates controllable isolation device91 based on information and command signals exchanged with generatorcontroller 40 to control transitions between the dual mode stand-alone(SA) operating modes and the dual mode grid-connect (GC) operatingmodes. During operation in the dual mode SA modes, isolation device 91is preferably opened to isolate protected load 92 from utilityelectrical power source 41. During operation in any of the dual mode GCmodes, isolation device 91 is preferably closed and connects protectedload 92 to utility electrical power source 41.

It should be recognized that the present invention also provides thecapability to operate turbogenerator/motor 10 in conventional SA and GCoperating modes in addition to the dual mode operation. That is, dualmode operation according to the present invention provides automatic orsemi-automatic transitions between operation in the SA modes andoperation in the GC modes. Dual mode controller 90 and/or generatorcontroller 40 may also be configured manually to operate in either theSA modes or the GC modes without automatic transitions. Control ofisolation device 91 may vary depending upon whether dual mode operationhas been selected and configured as opposed to one or the otheroperating modes. Alternatively, under dual mode control, isolationdevice 91 is opened in SA modes and starting power must be provided byan ancillary or auxiliary storage device, such as a battery.

Reconfiguration of generator controller 40 according to some embodimentsfor dual mode control according to the present invention may includerotation sequence control. Rotation sequence control provides theability to manually or automatically select a positive (L1, L2, L3) ornegative (L1, L3, L2) rotation sequence for the three phase power outputproduced by generator control 40. Manual selection is only available inthe SA operating modes. Automatic rotation sequence control ispreferably provided by generator controller 40. In one embodiment, dualmode transitions to SA operating modes will examine an appropriatememory storage location to determine a desired rotation sequence for theSA modes. The system can preferably be configured to command a positivesequence, a negative sequence, or to provide the same sequence asdetected on the utility grid in GC mode prior to transitioning to SAmode. The utility grid rotation sequence is monitored and periodicallyupdated as required. If a valid rotation sequence has not been detected,a default value may be used. Use of the same rotation sequence in SAmode as that used in GC mode may prevent possible equipment damage forprotected load 92 due to phase sequence reversal when transferringbetween utility grid and stand-by power. A detailed description of therotation sequence feature of a dual mode controller according to thepresent invention is provided with reference to FIG. 4.

As illustrated schematically in FIG. 2, generator controller 40 alsoincludes an energy storage and discharge system 69 having an ancillaryelectric storage device 70 which is connected through controlelectronics 71. This connection is bi-directional in that electricalenergy can flow from ancillary electric storage device 70 to generatorcontroller 40, during turbogenerator/motor start-up for example, andelectrical energy can also be supplied from turbogenerator/motorcontroller 40 to ancillary electric storage device 70 during sustainedoperation to handle load transients as explained above. While ancillaryelectric energy device 70 is schematically illustrated as an electricstorage battery, other electric energy storage devices can be utilized.By way of example, these would include flywheels, high energy capacitorsand the like.

Energy storage and discharge system 69 is connected to controller 40across a voltage bus. Energy storage and discharge system 69 includespreferably includes an off-load device (not shown) and ancillary energystorage and discharge switching devices (not shown). The off-load devicemay include an off-load resistor with associated switching devices whichmay include a charge switching device and a discharge switching device,for example.

A PWM inverter within controller 40 is used to convert the permanentmagnet turbogenerator/motor output to utility power, either sixty hertz,three phase for operation in stand-alone operating modes, or as acurrent source device for grid-connect operating modes. According to thepresent invention, the dual mode controller communicates with controllogic to automatically transition between SA and GC modes. As such, thePWM inverter is controlled to automatically transition betweencontrolling voltage in SA modes to controlling current in GC modes. Ofcourse other implementations are possible and the present invention isnot limited to controlling voltage in SA modes and controlling currentin GC modes.

The reconfiguration or conversion of the PWM inverter to be able tooperate as a current source synchronous with the utility grid may beaccomplished in one embodiment according to the present invention byfirst stopping the PWM inverter. The AC output or the grid connect pointis monitored with a separate set of logic to detect the rotationsequence and reference phase angle of the utility grid. When the controllogic detects that the grid is fit for reconnection, it will wait for aconfigurable reconnection delay and then initiate a transfer to gridconnect mode. The transfer starts with the gradual resynchronization ofthe stand-alone angle (phase) reference to the grid angle reference.Once the system is resynchronized to the grid, it will be able to runthe stand-alone controls while the isolating device (motorized circuitbreaker or contactor) is closing. Once the isolating device has beenverified closed using an auxiliary contact, for example, the system willtransition directly from the stand-alone load state to the grid-connectload state. If grid connect operation is not requested, or if closureverification is not timely received (such as within 100 milliseconds,for example), the system may transition into a cool-down state. Ineither case, the auxiliary storage and discharge system is rechargedbefore the system is automatically shutdown.

Dual mode control according to the present invention may be used toprovide for momentary transitions between GC and SA operating modes. Toreduce the transition time and provide only a momentary break in powerto the protected load, the present invention utilizes the auxiliarypower storage and discharge system 69 to provide power during thetransition. To activate this option, auxiliary power storage anddischarge system 69 is operated in a grid-connected charging mode duringthe grid-connected load state to assure immediate and sufficient poweravailability. The turbogenerator/motor may be kept in a lit state tofacilitate more immediate transfers with speed controlled using theoff-load device. The lit transfer state provides additional time toinitialize the auxiliary power storage and discharge system 69. Using amotorized circuit breaker or contactor, the momentary break orinterruption in power could be as short as 100 milliseconds.Alternatively, using a series combination of an electronic contactor andmechanical contactor or circuit breaker, the break time could be reducedto less than a cycle using the high bandwidth digital communicationsinterface, for example (RS-485), between dual mode controller 90 andgenerator controller 40.

Ancillary electric storage device 70 can continue motoringturbogenerator 10 for a short time after a shutdown in order to cooldown turbogenerator 10 and prevent unwanted heat recovery fromrecuperator 15. By continuing the rotation of turbogenerator 10 forseveral minutes after shutdown, power head 13 will keep moving air andsweep heat away from permanent magnet generator 12 to keep heat in theturbine end of power head 13. Preferably, the present invention providesa cool-down mode and a warm-down mode depending upon the particularoperating conditions and whether it is desirable to quickly restart theturbine.

Battery switching devices associated with ancillary electric storagedevice controller 71 provide a bi-directional path between ancillaryelectric storage device 70 and generator controller 40. Ancillaryelectric storage device 70, which is preferably a bank of independentlyconnectable batteries, can provide energy when a sudden demand or loadis required and the gas turbine is not up to speed.

FIG. 3 is a block diagram illustrating additional details of a dual modecontroller for one embodiment of a dual mode turbogenerator/motor systemaccording to the present invention. According to the present invention,a dual mode controller allows a turbogenerator/motor to automaticallytransition from a grid connect operating mode to a stand alone operatingmode when a utility outage occurs. Parallel connected protective modesare then powered by the turbogenerator/motor (operating in stand alonemode) during the utility power outage. When utility power is restored,the dual mode controller can be set to automatically return theturbogenerator/motor to grid connect service. The dual mode controlleraccording to the present invention also allows the turbogenerator/motorto be used as an automatically dispatched standby generator.

The dual mode controller of the present invention may be used in variousapplications operating in a standby only mode or a standby/grid connectmode, for example. When operating in a standby only application, theturbogenerator/motor is used as a standby generator. During a utilityoutage, the turbogenerator/motor and parallel connected protected loadsare isolated from the grid and the turbogenerator/motor powers theprotected loads. When utility power is restored, theturbogenerator/motor automatically shuts down and the protected loadsreturn to utility power.

For standby/grid connect applications of the dual mode controller theturbogenerator/motor is normally operated in a grid connect mode toreduce power draw from the utility. During a utility outage, theturbogenerator/motor is used to power the parallel connected protectedloads. When utility power is restored, the protected loads return toutility power and the turbogenerator/motor automatically returns to gridconnect mode.

In one embodiment of the present invention, the dual mode controllerincludes user controls to select the available operating mode or modes.The primary user control is preferably a selector switch whichdetermines the overall dispatch mode for the turbogenerator/motor.Representative operating modes may include an automatic mode in whichthe turbogenerator/motor is automatically dispatched in grid connectmode when the utility lines are energized and automatically transitionsto stand alone mode during a utility outage. A turbogenerator/motorpower mode may be provided in which the turbogenerator/motor may bedispatched in stand alone mode only. The turbogenerator/motor output andparallel connected protected loads will be electrically isolated fromthe utility lines. A line power mode may be provided in which theturbogenerator/motor may be dispatched in grid connect mode only. Duringa utility outage, the turbogenerator/motor will shut down and theprotected loads not be powered.

The embodiment of a dual mode controller according to the presentinvention illustrated in FIG. 3 preferably includes three indicatorlights mounted on the dual mode controller. A load power light (notshown) is illuminated when the loads are energized by either theturbogenerator/motor or the utility grid. A line power light (LP2) isilluminated when the utility grid is energized. A turbogenerator/motorawake light (LP3) is illuminated when the turbogenerator/motor controlsare energized.

Two user controls mounted on the dual mode controller manage operationof the turbogenerator/motor. A power switch (S1) is preferably a keyedswitch with on and off positions in addition to a momentary compactposition. The contact status for the three contacts provided by powerswitch S1 are summarized in the table of FIG. 3. As illustrated, thedual mode controller (and thus the turbogenerator/motor and its parallelconnected protected loads) may be switched on or off using power switchS1. The momentary contact position may be used to awaken theturbogenerator/motor from its battery-saving “sleep” state.

A mode select switch (S2) is provided to select the allowed operatingmodes for the particular application of the turbogenerator/motor and itsparallel connected protected loads. A table summarizing the context forthe mode select switch is also provided in FIG. 3.

In operation, when power switch S1 is in the off position, contacts S1A,S1B, and S1C are open preventing operation of the turbogenerator/motor.When power switch S1 is in the on position or momentary contact awakeposition, contacts S1A and S1B are closed which allows operation of theturbogenerator/motor provided additional contacts are in the appropriatestate as described below. In the momentary contact awake position,contact S1C is closed and the turbogenerator/motor controls are poweredup by the transient battery system.

When mode switch S2 is in the “line” position, contact S2A is open andcontact S2B is closed. This allows the turbogenerator/motor to beoperated only in the grid connect mode. If there is a utility outage,the turbogenerator/motor will shut down. When the mode switch is in theauto position, contacts S2A and S2B are closed. In this mode, theturbogenerator/motor will be automatically dispatched in grid connectmode when the utility lines are energized and automatically transitionedto stand alone mode during a utility outage. When the mode switch S2 ispositioned in the “turbine” mode, contact S2A is closed while contactS2B is open. In this mode, the turbogenerator/motor may be operated onlyin stand alone configuration. The turbogenerator/motor output will bephysically isolated from the utility lines.

The following sequence of events provides a representative automatictransfer from grid connect (GC) mode to stand alone (SA) mode.

1. Utility outage begins. The under voltage relay in the dual modecontroller senses the loss of grid voltage and opens (ref. contacts UV1,L2).

2. Power to the control relays (R1, L3 and R2, L2) is removed, openingtheir respective contacts.

3. The R2 contacts open (L1), removing power from the main isolationcontactor coil (M1). The isolation contactor opens, isolating theturbogenerator/motor and protected loads from the utility lines.

4. A set of auxiliary contacts on the isolation contactor also open(M1A, L5) and turbogenerator/motor GC operation is disabled. Another setof auxiliary contacts close (M1B, L6) which, if S1B, S2A, and R1 areclosed, closes the turbogenerator/motor's SA enable contacts, permittingSA operation.

5. The turbogenerator/motor initiates an orderly shutdown (warm-down).(This shutdown may have been triggered by the protective relay functionsof the turbogenerator/motor, before the under voltage relay tripped.)

6. The turbogenerator/motor “Not Load” relay closes “not Load”, 11 and12, L2), which is of no consequence at this particular point in thesequence.

7. The turbogenerator/motor completes its warm-down cycle (4-6 minutesdepending on ambient temperature) and enters and GC standby state.Immediately, since the GC enable contacts are open (L5), it enters thedisabled state. Again, immediately, since the SA enable contacts areclosed (L6), it enters the SA standby state and the “Not SA” relaycontacts (13 and 14, L2) open.

8. If the dispatch mode active for stand alone calls for it, and theauto restart feature in the turbogenerator/motor is enabled, itinitiates a restart in SA mode, which will require approximately twominutes.

9. After the stand along start sequence is completed, the auto loadfeature of the turbogenerator/motor, if enabled, connects theturbogenerator/motor output to the protected loads and the “Not Load”relay (11 and 12, L2) opens, preventing the isolation contactor fromconnecting the grid to the turbogenerator/motor and protected loads foras long as the turbogenerator/motor is in the load state.

The following representative sequence of events occurs during anautomatic transfer from grid connect mode to stand alone mode.

1. Utility outage ends. Following a time delay set in the under voltagerelay, its contacts close (UV1, L2).

2. If S2B and S1A are closed, power to the SA disable relay coil (R1) isrestored, opening its contacts (R1, L6), disabling SA operating.

3. The turbogenerator/motor initiates an orderly shutdown. Theturbogenerator/motor state changes from load to not load, closing the“Not Load” relay contacts in the turbogenerator/motor (11 and 12, L2).

4. The isolation contactor pilot relay coil (R2, L2) energizes, closingits contacts (R2, L1). The isolation contactor (M1) closes connectingthe utility lines to the protected load(s) and the turbogenerator/motor(elapsed time since return of utility—approximately 10 seconds).

5. After the turbogenerator/motor completes a battery recharge phase anda warm-down cycle (up to 32 minutes depending on ambient temperature andthe state of the battery charge), it enters the SA standby state.Immediately, since the SA enable circuit (L6) is open, it enters thedisable state. The “Not SA” relay contacts open (13 and 14, L2). Againimmediately, since the GC enable circuit is closed (L5), it enters theGC standby state.

6. If the dispatch mode active for grid connect calls for it, and theauto restart feature in the turbogenerator/motor is enabled, itinitiates a restart in GC mode, which will require approximately twominutes.

FIG. 4 is a diagram illustrating operation of rotation sequence controlfor one embodiment of a dual mode controller for a turbogenerator/motoraccording to the present invention. Block 150 of FIG. 4 representsdetermining whether the system is currently operating in grid connect ordual mode-grid connect operating mode. When operating in the gridconnect or dual mode grid-connect state, the system has the ability tomeasure the rotation sequence of the utility grid as represented byblock 152. This allows the same rotation sequence to be used in the gridconnect and stand-alone modes as described above. This will preventpossible equipment damage for the protected load due to a potentialphase sequence reversal when transferring between grid connect andstand-alone operating modes.

Block 154 compares the measured rotation sequence with a previouslystored value which is preferably stored in a persistent memory. If thecurrently measured grid rotation sequence differs from the previouslystored value, and if the current value is valid, i.e. can be measured,block 156 updates the rotation sequence value by storing the currentvalue in the persistent memory. If a valid measurement of the gridrotation sequence has not yet been made, an appropriate value is stored,such as zero for example. In one preferred embodiment of the presentinvention, a first value is used to indicate a positive rotationsequence (L1, L2, L3) while a second value is used to indicate anegative rotation sequence (L1, L3, L2).

The present invention provides the capability to send a rotationsequence command or query as represented by block 158. If operating inthe grid connect or dual mode grid-connect state, the rotation sequencecannot be changed since the system is connected to the utility grid. Assuch, the sequence command is treated as a query and the last measuredrotation sequence is reported as represented by block 160. Preferably,queries and/or commands may be transmitted to and from the system overan RS-232 communications link.

If the system is not operating in a grid connect operating mode asdetermined by block 150, block 170 determines whether the system isoperating in stand-alone or dual mode stand-alone. Preferably, each timethe system enters the stand-alone state machine, a stored defaultrotation sequence will be retrieved as represented by block 172. Thiscan occur either at power-up, or on transition from the grid-connecteddual mode state. Block 174 of FIG. 4 sets the rotation sequence to theretrieved default value. Block 176 determines whether the default valuecorresponds to positive or negative rotation. If positive or negativerotation is indicated, the appropriate rotation sequence command is sentto the inverter as indicated by block 178. The present invention alsoallows a default value corresponding to the current or last-measuredrotation sequence of the utility grid. If the default value correspondsto the grid rotation sequence, control passes to block 180 where thelast-measured and/or stored grid sequence is retrieved. Block 182determines whether the stored grid sequence is valid. If not, the systemgenerates a fault as indicated by block 184. An invalid grid rotationsequence may result if the grid rotation sequence has not yet beenmeasured, for example. If the retrieved grid rotation sequence is valid,an appropriate sequence command is sent to the inverter as indicated byblock 178.

In addition to providing a default rotation sequence for operation indual mode stand-alone configuration, the present invention also providesrotation sequence control outside of dual mode operation. When notoperating in dual mode, and currently in the stand-alone mode asindicated by block 186, block 188 awaits receipt of a rotation sequencecommand from the user. In one preferred embodiment, the rotationsequence command is provided via an RS-232 communications link to allowthe user to set the rotation sequence. Block 190 determines whether thereceived rotation sequence command is valid. If the command includes anappropriate parameter to set the sequence to positive or negative, thecorresponding sequence command is sent to the inverter as indicated byblock 178. Otherwise, if block 190 determines the command is invalid, afault is generated as indicated by block 184.

FIG. 5 is a simplified flowchart illustrating dual mode control of aturbogenerator/motor during a transition from a grid-connect mode to astand-alone mode according to the present invention. Functions enclosedby dashed lines 198 are preferably performed by the dual modecontroller/sub-assembly while the remaining functions illustrated arepreferably performed by the turbogenerator/motorcontroller/sub-assembly. The dual mode controller monitors the utilitygrid and, when necessary, operates an isolation device to separate orisolate the turbogenerator/motor and parallel connected loads usingeither a contactor or a motorized circuit breaker as described above.For implementations which utilize an external dual mode controller, theexternal controller preferably interfaces with the generator controllerof the turbogenerator/motor by exchanging signals for battery start andsystem start/stop inputs in addition to standby, run, load, and faultstate indicators. An external dual mode controller may also communicategrid-connect enable, stand-alone enable, and synchronization signals tothe generator controller. This enables an external dual mode controlsystem to start the turbogenerator/motor system in the event that it wasnot operating in the grid-connect load state when a grid interruptionoccurred.

If the system detects a fault as represented by block 200, a shutdown202 may be initiated. Depending upon the severity of the fault, shutdown202 may be performed using a cool down 204 routine or a warm down 206routine. Block 208 monitors the utility grid and provides an appropriateindication of grid loss or a grid fault. Various grid monitoring systemsand methods may be used without departing from the spirit or scope ofthe present invention to determine the availability or condition of theutility grid power.

When a grid interruption is detected by block 208, block 210 opens theisolation device to isolate the turbogenerator/motor and parallelconnected protected load from the utility grid. An appropriate statusindicator is set to indicate that the grid-connect mode is no longerenabled as represented by block 212. An auxiliary contact or similardevice is used to confirm that the isolation device has actually openedas commanded as represented by block 214. The stand-alone enable signalis then asserted as represented by block 216. Block 218 determineswhether the turbogenerator/motor is in the run state. Depending upon theparticular application, transition from the grid-connect load state tothe stand-alone load state may take several minutes, during which timethe protected load will receive no power. In this case, theturbogenerator/motor is not in the run state when transitioning from thegrid-connect mode. As such, block 220 commands a battery start and block222 commands a system start. The turbogenerator/motor is operated as amotor with a variable speed drive function provided by the PWM inverteruntil the turbogenerator/motor reaches self-sustaining operation asindicated by block 218.

Block 224 removes the battery start signal and block 226 determines theappropriate rotation sequence for the three-phase power to be suppliedby the generator and PWM inverter. The inverter is then appropriatelyreconfigured to supply the load and control the voltage as representedby block 228. The auxiliary energy storage device, or battery bank inthis example, is then serviced as represented by block 230. Dependingupon the particular operating conditions, this may include performing anequalization charge 232, a top-up charge 234, or a recharge 236. Propermaintenance of the auxiliary electrical storage and discharge systemprovides for faster transitions in dual mode and lowers the risk of thestorage device becoming depleted when the system is operating in agrid-connected mode. The grid is continually monitored to determinewhether a transition back to grid-connect mode may be performed,assuming the dual mode controller is appropriately configured, asrepresented by block 238.

FIG. 6 provides a simplified flowchart illustrating one embodiment of adual mode controller operating without resynchronization and with stopand automatic restart (Dual Mode 1) during a transition from astand-alone mode to a grid-connect mode according to the presentinvention. FIG. 6 assumes the system is operating in stand-alone modebeginning with block 238 of FIG. 5 where the grid is monitored todetermine if reconnection is appropriate. If reconnection is appropriateas indicated by block 238, block 240 removes the stand-alone (SA) enablelogic signal. Block 242 verifies that the stand-alone output has ceased.Block 244 closes the isolating device to reconnect theparallel-connected protected load with the utility grid. Block 246asserts or energizes the grid connect (GC) enable signal once theisolating device is verified closed using an auxiliary contact and theprotected load is supplied from the grid as indicated at block 248. Ifgrid-connect operation is not requested, or if the closure of theisolating device cannot be verified within a predetermined time, a faultwill be generated and the system will transition to a cool down mode(not shown). In either case, the auxiliary electrical energy storage anddischarge system is serviced as represented by block 250. This mayinclude an equalization charge 252, top-up charge 254, or recharge 256,for example. A cool down is then performed as represented by block 258.Block 260 performs an automatic restart in grid connect mode andsupplies power to the utility grid as represented by block 262.

FIG. 7 provides a simplified flowchart illustrating one embodiment of adual mode controller operating without resynchronization and withoutstop and automatic restart (Dual Mode 2) during a transition from astand-alone mode to a grid-connect mode according to the presentinvention. FIG. 7 assumes the system is operating in stand-alone modebeginning with block 238 of FIG. 5 where the grid is monitored todetermine if reconnection is appropriate. Blocks 240, 242, 244, and 246perform functions as described above with reference to FIG. 6. Afterasserting the grid-connect (GC) enable signal in block 246, power issupplied to the utility grid and to the protected load as represented byblock 264. The auxiliary electric energy storage device is then servicedas represented by blocks 250, 252, 254, and 256 as described above.

FIG. 8 provides a simplified flowchart illustrating one embodiment of adual mode controller operating with resynchronization and without stopand automatic restart (Dual Mode 3) during a transition from astand-alone mode to a grid-connect mode according to the presentinvention. FIG. 8 assumes the system is operating in stand-alone modebeginning with block 238 of FIG. 5 where the grid is monitored todetermine if reconnection is appropriate. When reconnection to the gridis appropriate in this mode, the grid rotation sequence and phasereference angle are detected as represented by block 266. The generatorcontroller then adjusts the rotation sequence and/or phase referenceangle if required to synchronize to the grid as represented by block268. Operation then continues with blocks 240, 244, 246, 264, 250, 252,and 254 as described above. However, the output from the generatorcontroller is not interrupted in the transition between stand-alone andgrid-connected modes.

FIG. 9 provides a state transition diagram illustrating representativestates for operation of a dual mode control system in the grid-connectmode, including states for transitioning from grid-connect mode tostand-alone mode according to the present invention. As will beappreciated by those of skill in the art, the state transition diagramsof FIGS. 9 and 10 do not necessarily represent all possible states ortransitions between states but are provided to illustrate the mostcommon states for operation of a turbogenerator/motor in arepresentative application utilizing dual mode control according to thepresent invention. Various states of the automatic dual mode may becommon to a corresponding manual transition operating mode active whenthe system is not operating under dual mode control. For example,various states of FIG. 9 may be common to the manual grid-connect modeand automatic or semiautomatic dual mode operation.

Initial power-up 270 transitions to standby 272 upon passing variouspower-on self tests (POSTs). Grid-connect standby 272, upon meetingappropriate exit conditions, may transition to standby recharge state274, prepare-to-start state 276, or to stand-alone standby state 322(best illustrated in FIG. 8). Transition from standby state 272 tostandby recharge state 274 is performed if the auxiliary storage systemindicates a charge is needed. Once charged, the state may return tostandby state 272. If a start command is received while in state 272 or274, prepare-to-start state 276 is entered. In a normal start-up, thestates proceed through bearing liftoff 278, open loop light-off 280, andclosed loop acceleration 282 until the turbogenerator/motor reaches anappropriate operating speed and temperature which allows it to enter theload state 284. Various conditions occurring between theprepare-to-start state 276 and load state 284 may result in appropriatecorrective action such as a cool down 294, restart 296, or shutdown 298.Depending on the particular anomaly and corresponding severity, thesystem may reenter standby 272 from shutdown state 298, may generate afault 300, or for some conditions may result in entering a disable state304, such as when an appropriate status or fault code is indicated asrepresented by block 302. Once in fault state 300, some faults may becleared and the system returned to standby state 272 after anappropriate reset period. Likewise, after the system enters shutdownstate 298 and the shaft stops rotating, the system may return to standbystate 272.

Under normal operating conditions, dual mode grid-connect load state 284may transition to stand-alone transfer state 286 when a grid fault isdetected or when the dual mode controller status flags or enable signalsare inconsistent with grid-connected operation. This may occur anytimethat the stand-alone enable signal is set, the grid-connect enablesignal is not set, or when a grid fault is received. Upon entry intostate 286, the PWM inverter is preferably disabled and reset. Theauxiliary power storage and discharge system is enabled and set toregulate the DC bus. After resetting the PWM inverter, the appropriateparameters are loaded to reconfigure the PWM inverter as agrid-connected AC current source. A stand-alone auto-restart timer isthen reset to zero. If the grid-connect enable signal returns to a validstate, state 286 transitions back to the load state 284. If thestand-alone mode logic signal is set and a valid start command has beenreceived, the system transitions to stand-alone run mode 334 (bestillustrated in FIG. 8). In a fully automatic mode, a transition fromstate 286 to 334 is performed if an auto restart enable flag is set anda delay timer has expired. In a semi-automatic mode, the system may waitfor a start command from the user/operator. Once the auxiliary powerstorage and discharge system has entered the regulate DC bus mode, thePWM inverter is loaded with the stand-alone parameters to reconfigurethe inverter for stand-alone operation where the voltage is controlled.The system then enters the stand-alone run state 334.

From transfer state 286, various conditions may cause the system toenter battery assisted cool down state 288, a first warm down state 290,or a second warm down state 292. For example, if a user selectabletransition time is exceeded while in state 286, the system maytransition to battery assisted cool down state 288. Likewise, if a stopcommand is received, the system transitions to state 288. In each of thecool down and warm down states 288, 290, 292, and 294, theturbogenerator/motor fuel system is shut down and the turbogeneratorbegins to decelerate. Battery assisted cool down state 288 may supplypower to the motor to maintain circulation of air through theturbogenerator and remove heat from the recuperator as described above.The system may transition to the stand-alone cool down state 336 (bestillustrated in FIG. 10), or to the warm down states 290, 292, or shutdown state 298.

FIG. 10 provides a representative state transition diagram illustratingoperation of one embodiment for a dual mode control system in thestand-alone mode including states for transitioning from the stand-alonemode to the grid-connect mode according to the present invention. Aswith the state transition diagram of FIG. 9, FIG. 10 illustrates themost common states and transitions for a representative application.However, the present invention is independent of the particular statesand/or transitions including entry and exit conditions which areprovided for ease of illustration and explanation only. Likewise,various states illustrated in FIG. 10 may be common to a manual orsemi-automatic stand-alone mode which may require some intervention froman operator/user to complete the transition between operating modes.

A typical operating sequence may proceed through power-up state 320 tostandby state 322 after passing the POST. Transition from standby state322 may proceed to grid-connect standby state 272 (FIG. 9) during a modetransition between stand-alone and grid-connect modes. A typicalstart-up sequence proceeds through prepare-to-start state 326 where theturbogenerator/motor is operated as a motor with increasing shaft speed,bearing lift off state 328 where the rotational speed is sufficient forproper operation and cooling of the foil bearings, open loop light-offstate 330 where fuel is supplied to the combustor and the ignitor isenergized to begin combustion, closed loop acceleration state 332, andrun or load state 334. The run or load state 334 may be entered directlyfrom the stand-alone transfer state 286 (FIG. 9). Various operatingconditions may result in a system cool down 336, shutdown 346, or warmdown 344, 350. Severe faults may result in entering the system disablestate 356. After an appropriate reset period, some faults resulting infault state 358 may be cleared and the system returned to standby state322. Likewise, after the system enters shutdown 346 and the shaft stopsrotating, the system may return to standby state 322.

Stand-alone run state 334 is the branch-in point for automatictransitions from the grid-connect to SA transfer state 286 (FIG. 9). Ifthe system is appropriately configured, it will transition immediatelyinto stand-alone load state 334. Additional requirements are describedwith respect to state 286 of FIG. 9.

Recharge state 338 of FIG. 8 can be entered from grid-connect transferstate 340 or run state 334. From state 334, if the stand-alone andgrid-connect enable signals are in an invalid or inconsistent state forstand-alone operation, i.e. the stand-alone enable signal is not set orthe grid-connect enable signal is set, or when an appropriate level ofsystem fault status is encountered, the system transitions to rechargestate 338. The system can exit from recharge state 338 into stand-alonegrid-connect transfer state 340, warm down state 344, cool down state336, or back to run state 334 depending upon the particular operatingconditions.

Stand-alone grid-connect transfer state 340 disables the PWM inverterand resets any faults. The auxiliary power storage and discharge deviceis controlled as for the stand-alone recharge state 338. The generatorcontroller controls the shaft speed of the turbogenerator and fuelcontrol is set the same as in the stand-alone recharge state 338. Atransition from recharge state 338 to transfer state 340 loads the PWMinverter with appropriate grid-connect parameters to reconfigure theconverter to transition from stand-alone voltage control to grid-connectAC current control, however, the inverter is not enabled at this point.The system may transition from transfer state 340 to grid-connect loadstate 284 (FIG. 9) once the inverter has synchronized to the grid and avalid start command is received or auto restart is enabled.

As such, the present invention provides systems and methods for dualmode control of a turbogenerator/motor to transition between a utilitygrid connected mode and a stand alone operating mode. Appropriaterotation sequence and synchronization provides the capability formomentary transitions between a stand-alone, isolated mode and agrid-connected mode. An auxiliary energy storage and discharge systemprovides the necessary additional power for load transients, such asthose occurring during transitions between modes.

While embodiments of the invention have been illustrated and described,it is not intended that these embodiments illustrate and describe allpossible forms of the invention. Rather, the words used in thespecification are words of description rather than limitation, and it isunderstood that various changes may be made without departing from thespirit and scope of the invention.

What is claimed is:
 1. A method for controlling a turbogenerator andsuppling power to a load, comprising: (a) generatingvariable-voltage/variable-frequency electrical power with a permanentmagnet turbogenerator the variable-voltage and the variable-frequency ofthe generated electrical power being dependent upon the output powerlevel of the permanent magnet turbogenerator; (b) converting thevariable-voltage/variable-frequency electrical power tofixed-voltage/fixed-frequency electrical power with a generatorconverter capable of operating in one of a grid-connected mode and astand-alone mode; (c) supplying fixed-voltage/fixed-frequency electricalpower from the generator converter and the permanent magnetturbogenerator to at least one of a utility electrical power source anda protected load when the generator converter is operating in thegrid-connected mode; (d) monitoring the utility electrical power source;(e) automatically disconnecting the generator converter and thepermanent magnet turbogenerator from the utility electrical power sourceupon detection of a fault in the utility electrical power source; (f)automatically switching the operating mode of the generator converterfrom grid-connected mode to stand-alone mode upon detection of a faultin the utility electrical power source; and (g) supplyingfixed-voltage/fixed-frequency electrical power from the generatorconverter and the permanent magnet turbogenerator to the protected loadwhen the generator converter is operating in the stand-alone mode. 2.The method of claim 1, wherein step (d) comprises: monitoring a phaserotation sequence of the utility electrical power source.
 3. The methodof claim 1, wherein step (d) comprises: monitoring a phase referenceangle of the utility electrical power source.
 4. The method of claim 1,further comprising: (h) detecting that the utility electrical powersource is acceptable for reconnection of the generator converter and thepermanent magnet turbogenerator; and (i) reconnecting the generatorconverter and the permanent magnet turbogenerator to the utilityelectrical power source.
 5. The method of claim 4, further comprising:(j) prior to step (i), adjusting the phase angle of thefixed-voltage/fixed-frequency electrical power from the generatorconverter and the permanent magnet turbogenerator so that the phaseangle of the fixed-voltage/fixed-frequency electrical power issynchronized with the phase angle of the utility electrical powersource.
 6. The method of claim 5, further comprising: (k) switching theoperating mode of the generator converter from stand-alone mode togrid-connected mode.
 7. A method for controlling a turbogenerator andsupplying power to a load, comprising: (a) providing power to a loadfrom a utility electrical power source; (b) providing power to theutility electrical power source from a turbogenerator; (c) monitoringthe utility electrical power source; (d) disconnecting the load and theturbogenerator from the utility electrical power source upon detecting afault in the utility electrical power source; and (e) after step (d),providing power from the turbogenerator to the load.
 8. The method ofclaim 7, wherein step (c) comprises: monitoring a phase rotationsequence of the utility electrical power source.
 9. The method of claim7, wherein step (c) comprises: monitoring a phase reference angle of theutility electrical power source.
 10. The method of claim 7, furthercomprising: (f) upon detecting that the utility electrical power sourceis acceptable for reconnection of the turbogenerator, reconnecting theturbogenerator to the utility electrical power source and providingpower to the utility electrical power source from the turbogenerator.11. The method of claim 7, further comprising: (g) prior to step (f),adjusting the phase angle of the electrical power of the turbogeneratorso that the phase angle of the electrical power is synchronized with thephase angle of the utility electrical power source.
 12. A method forcontrolling a turbogenerator and suppling power to a load, comprising:(a) generating electrical power with a turbogenerator, the electricalpower having a voltage and a frequency that is dependent upon the outputpower level of the turbogenerator; (b) converting the electrical powerto fixed-voltage/fixed-frequency electrical power with a generatorconverter capable of operating in one of a grid-connected mode and astand-alone mode; (c) supplying fixed-voltage/fixed-frequency electricalpower from the generator converter and the permanent magnetturbogenerator to at least one of a utility electrical power source anda protected load when the generator converter is operating in thegrid-connected mode; (d) monitoring the utility electrical power source;(e) automatically disconnecting the generator converter and thepermanent magnet turbogenerator from the utility electrical power sourceupon detection of a fault in the utility electrical power source; (f)automatically switching the operating mode of the generator converterfrom grid-connected mode to stand-alone mode upon detection of a faultin the utility electrical power source; and (g) supplyingfixed-voltage/fixed-frequency electrical power from the generatorconverter and the permanent magnet turbogenerator to the protected loadwhen the generator converter is operating in the stand-alone mode. 13.The method of claim 12, further comprising: (h) detecting that theutility electrical power source is acceptable for reconnection of thegenerator converter and the permanent magnet turbogenerator; and (i)reconnecting the generator converter and the permanent magnetturbogenerator to the utility electrical power source.
 14. The method ofclaim 13, wherein step (d) comprises: monitoring a phase reference angleof the utility electrical power source.
 15. The method of claim 14,further comprising: (j) prior to step (i), adjusting the phase angle ofthe fixed-voltage/fixed-frequency electrical power from the generatorconverter and the permanent magnet turbogenerator so that the phaseangle of the fixed-voltage/fixed-frequency electrical power issynchronized with the phase angle of the utility electrical powersource.
 16. The method of claim 15, further comprising: (k) switchingthe operating mode of the generator converter from stand-alone mode togrid-connected mode.
 17. The method of claim 12, wherein step (d)comprises: monitoring a phase rotation sequence of the utilityelectrical power source.